1. Field of the Invention
The present invention relates to a toothed belt used as a timing belt or a synchronous belt, and more particularly relates to a toothed belt used as a power transmission belt for transmitting the rotational drive force of an engine of an automobile or other vehicle from a crankshaft to a cam shaft, balancer shaft, drive shaft of a fuel injection pump, etc.
2. Description of the Related Art
A conventional toothed belt is provided with a toothed rubber layer. Teeth and bottoms are alternately formed on one surface of the toothed rubber layer. A back rubber layer is integrally provided at the other surface of the toothed rubber layer. Cords are embedded at the interface between the toothed rubber layer and the back rubber layer. In recent years, along with the improvement of the performance of engines of automobiles and other vehicles, the speed of crank shafts has risen and a much greater load has been placed on the toothed belts used for driving the cam shaft, injection pump shaft, and other auxiliary equipment shafts. Therefore, breakage of teeth has become faster.
As a means to prevent such premature tooth breakage accompanying increases in load, as described in Japanese Unexamined Patent Publication No. 10-89418, it has been considered to strengthen the rigidity of the toothed rubber layer by mixing into the toothed rubber layer aramid staple fibers oriented in one direction. The bondability with the rubber ingredient of the toothed rubber layer when intermixing aramid staple fibers, however, is not sufficient. If the toothed belt is used under high load conditions over a long period, the problem arises of breakage of the teeth due to cracks from the interfaces between the aramid staple fibers and the rubber ingredient.
Therefore, an object of the present invention is to provide a toothed belt configured to solve the problem of the premature breakage of teeth.
The toothed belt of the present invention is provided with a toothed rubber layer. The toothed rubber layer has teeth and bottoms alternately formed on one surface. A back rubber layer is integrally joined to the other surface of the toothed rubber layer. A plurality of cords are interposed at the interface between the toothed rubber layer and the back rubber layer. These cords extend along the length direction of the toothed belt and are arrayed along the width direction. According to the present invention, a large number of modified nylon microfibers are mixed and regularly oriented in the toothed rubber layer in the toothed belt as a whole and the modified nylon microfibers are fine denier nylon fibers obtained by graft polymerization with a polyolefin.
In the present invention, a fiber length LF of the modified nylon microfiber may be not more than about 4000 xcexcm, preferably not more than about 1000 xcexcm. Further, the fiber diameter DF of the modified nylon microfiber may be not more than about 1.5 xcexcm, preferably not more than about 1.0 xcexcm. Further, the value of the ratio of the fiber length LF to the fiber diameter DF (LF/DF) may be not less than 10, preferably in the range of 500 to 1500.
In the present invention, the nylon fibers are preferably made of Nylon 6 and the polyolefin is preferably made from polyethylene.
In the present invention, it is possible to intermix about 10 to about 40 parts by weight of modified nylon microfibers in 100 parts by weight of the rubber stock of the toothed rubber layer. Preferably, about 25 parts by weight of modified nylon microfibers are intermixed.
In the present invention, the stock rubber used for the toothed rubber layer and the back rubber layer may be a hydrogenated nitrile rubber with a hydrogenation rate of at least 91% and a peroxide-based vulcanizing agent may be added to the stock rubber.
Preferably, the modified nylon microfibers are oriented in the length direction of the toothed belt. In this case, the modified nylon microfibers are oriented along contours at regions near alternating contours of the teeth and the bottoms of the toothed belt layer and are oriented substantially perpendicular to the surface of the back rubber layer at the center regions of the teeth of the toothed rubber layer. On the other hand, the modified nylon microfibers may be oriented in the width direction of the toothed belt.
In the present invention, preferably, fabric is provided to cover the teeth and the bottoms of the toothed rubber belt. The fabric is comprised of an elastic composite yarn extending along the longitudinal direction of the toothed belt and a non-elastic yarn extending along the width direction of the toothed belt. The elongation of the fabric when elongated to breakage is made about 30 to about 80% of the original length. Further, the fabric may be treated by a resorcinol formaldehyde latex solution. Alternatively, the fabric may be preformed.
In the present invention, the cords may be formed from high strength glass fibers and the cords may be arrayed at intervals of about 0.17 to about 0.28 mm. Further, the cords may be formed from aramid fibers and the cords may be arrayed at intervals of about 0.25 to about 0.36 mm. Preferably, the cords are treated by a resorcinol formaldehyde latex solution, an overcoat layer of a rubber paste solution is formed on the cords, and preferably a cashew-modified phenol resin layer is formed on the overcoat layer.